U.S. Pat. Nos. 5,428,123; 5,589,563; and 5,756,632, issued Jun. 27, 1995; Dec. 31, 1996 and May 28, 1998, respectively, together disclose a type of material and a way of engineering this material to have a particular permselectivity. All three of these patents are hereby incorporated by reference as if set forth in their entirety herein. When efforts were made, however, to use this material in an indwelling glucose sensor application it was found that the requirement for high oxygen and glucose permeability was at conflict with the requirement for structural strength and integrity after exposure to an oxidative environment. More specifically it was found that when the material was made sufficiently oxygen permeable it became too weak and tended to break apart on the sensor, after being placed in the body's interstitial fluid for more than a few hours.
Within biological solutions such as blood or interstitial fluid there exist a number of reactive materials and enzymes that may bring about cleavage of the polymer's molecular chains and thus result in loss of membrane or fiber strength and integrity. Some of the reactive materials and enzymes that may bring about polymer degradation and cleavage include small molecules such as superoxide (O2−) and acids, and enzymes such as proteases and oxidases that react with the various types of linkages in the polymer. This loss of membrane or fiber integrity is deleterious to applications which depend on the permselectivity of the polymeric material and the exclusion of solids and larger biological molecules, such as the detection of the levels of glucose within the body fluids of a living human body.